1. Field of the Art
The present invention relates in general to an anti-skid pressure control device to control a braking pressure in brake cylinders in a hydraulic braking system for an automotive vehicle, in order to prevent an excessive slip of drive wheels of the vehicle on the road surface upon brake application. More particularly, the invention is concerned with a relief valve used in such an anti-skid pressure control device.
2. Related Art Statement
An example of such a hydraulic braking system for an automotive vehicle is known, according to Japanese Patent Application laid open in 1981 under Publication No. 56-128251, which comprises: (a) a reservoir having a housing, and a piston received in the housing slidably movably over a limited distance and cooperating with the housing to define a fluid chamber of a variable volume in which a brake fluid is stored; (b) a solenoid valve disposed in a primary fluid passage connecting a master cylinder and a brake cylinder to feed the brake fluid from the master cylinder to the brake cylinder, the solenoid valve being selectively placed in one of a first operating position for communication of the brake cylinder with the master cylinder, and a second operating position for communication of the brake cylinder with the reservoir; (c) a controller for monitoring an amount of slip of a drive wheel of a vehicle on the road surface, and controlling the solenoid valve so as to control the braking pressure in the brake cylinder at an optimum level; (d) a pump for pumping the brake fluid from the reservoir and returning the pumped fluid via a pump passage to the primary fluid passage at a point thereof between the master cylinder and the solenoid valve; (e) a shut-off valve in a portion of the primary fluid passage between the master cylinder and the above-identified point, the shut-off valve being closed to prevent a flow of the brake fluid from the pump passage toward the master cylinder at least while the brake fluid is being fed by the pump; (f) and a relief valve disposed between the outlet of the pump and the reservoir, to permit the brake fluid from the pump to be returned into the reservoir when the delivery pressure of the pump exceeds a preset level.
The hydraulic braking system with the anti-skid pressure control device constructed as described above, does not suffer a so-called "kick-back" phenomenon of a brake pedal or similar manually-operated member, which is conventionally encountered due to a return flow of the brake fluid from the pump back into the master cylinder via the primary fluid passage. The elimination of the "kick-back" phenomenon leads to an improved operating feel of the brake pedal.
In the hydraulic braking system of the type indicated above, however, the reservoir may be filled with the brake fluid, i.e., the brake fluid from the brake cylinder may fill the maximum volume of the fluid storage chamber of the reservoir with its piston located at its stroke end. In this condition, the reservoir is not more able to accommodate the brake fluid from the brake cylinder, causing the anti-skid pressure control device to fail to effect an intended control of the braking pressure in the brake cylinder. For instance, the reservoir will be filled when a leakage flow occurs in the solenoid valve while a braking pressure is applied to the brake cylinder repeatedly or intermittently for repeated brake application without an anti-skid pressure control. In this event, the leakage flow from the solenoid valve fills the reservoir. The reservoir may be filled, also when the brake fluid is discharged from the brake cylinder as a result of an anti-skid pressure control of the brake cylinder, and the discharged fluid is fed into the reservoir in a partially-filled condition.
To solve the above-indicated inconveniences, the assignee of the present application proposed a solution which is disclosed in Japanese Patent Application No. 59-64269 (filed in 1984), wherein a second relief valve is disposed between the outlet side of the pump and the master cylinder, in addition to the previously indicated relief valve (referred to as first relief valve). Further, the proposed solution uses an arrangement in which the piston of the reservoir has a predetermined intermediate position part way through its full stroke, in which the volume of the fluid storage chamber of the reservoir is smaller by a given amount than its maximum volume corresponding to the stroke end position of the piston. The reservoir has means for restricting or completely blocking a flow of the brake fluid from the pump into the reservoir via the first relief valve when the reservoir piston is placed in the intermediate position.
In the proposed arrangement, the intermediate volume of the storage chamber of the reservoir corresponding to the intermediate position of the piston may be filled with the brake fluid upon leakage flow of the fluid from the solenoid valve, for example. However, the storage chamber will not be filled with the brake fluid discharged from the brake cylinder during an anti-skid pressure control operation, because the discharged brake fluid is returned to the master cylinder via the second relief valve. Thus, the proposed arrangement is free from the previously indicated problem that the reservoir may be filled and unable to accommodate the brake fluid discharged from the brake cylinder. Thus, the proposed anti-skid pressure control device is improved in operating reliability.
In the meantime, however, there is another problem in connection with the proposed anti-skid pressure control device. Stated more particularly, the second relief valve, which permits a return flow of the fluid from the pump passage into the master cylinder, is opened while a braking pressure is applied to the brake cylinder. Therefore, the braking pressure exists in the master cylinder when the second relief valve is opened. Accordingly, the pressure at which the second relief valve is opened is affected by the master cylinder pressure. Namely, the maximum pressure in the pump passage is affected by the maximum braking pressure generated by the master cylinder. In an ordinary relief valve, its valve member is forced against the valve seat not only by a biasing spring but also by a pressure in the valve chamber in which the valve member is accommodated. In this arrangement, the relief valve will not be opened until the pressure in the pump passage has been elevated to a level which is high enough to overcome the biasing force of the spring and the pressure in the valve chamber (viz., master cylinder pressure). Moreover, the first relief valve is usually designed to be opened at a pressure which is not lower than the maximum braking pressure generated in the master cylinder. On the other hand, the second relief valve is designed to be opened at a pressure higher than the opening pressure of the first relief valve, even while the master cylinder pressure is considerably low. Hence, when the master cylinder pressure is high, the second relief valve will be open only when the pressure in the pump passage is extremely high. Accordingly, the pump, master cylinder, shut-off valve, piping system, etc. must be designed so as to withstand a very high pressure, which leads to increased weight and cost of the braking system. Further, such a very high flud pressure will be applied to the master cylinder when the second relief valve is open, and will cause a "kick-back" trouble with the brake pedal. Although this "kick-back" phenomenon will happen very infrequently and give substantially no effect on the operating feel of the brake pedal, the occurrence of such an infrequent "kick-back" phenomenon may make the vehicle driver feel uneasy about the reliabilty of the braking system, particularly when the kick-back motion is large. For this reason, it is desired to minimize the kick-back phenomenon caused by the fluid return into the master cylinder.